The Mediterranean diet

Some diets have proven beneficial effects on health, such as the Mediterranean diet, MD. The Dietary Approaches to Stop Hypertension, DASH diet, the Baltic Sea diet, and the Okinawa diet. Those nutritional patterns have one thing in common, the frequent consumption of healthy foods.

The Mediterranean Diet is centred around fruits, vegetables, olive oil, nuts, legumes and whole grains. The diet is low in animal proteins, sugar, starch, and high in fibre, healthy fats, such as monounsaturated and polyunsaturated fatty acids, and antioxidant substances, such as polyphenols.

The adherence to the Mediterranean Diet has been linked to a large number of health benefits, including reduced mortality risk and the prevention of many diseases, such as cardiovascular diseases, diabetes mellitus, the metabolic syndrome, cancer, cognitive impairment, and depression.

As we saw during week two, a high-fibre intake gives place to short-chain fatty acid production through the fermentation of these fibres by anaerobic intestinal microbiota. In particular, a better adherence to the Mediterranean Diet was associated with significantly higher levels of total short-chain fatty acids. These compounds have been shown to exert multiple beneficial effects on mammalian energy metabolism.

In the cecum and large intestine, 95% of the produced short-chain fatty acids are rapidly absorbed by the colonocytes and then can be found in hepatic and peripheral blood.

It has been noted that some short-chain fatty acids, such as butyrate, can, by linking to specific receptors, help maintain the metabolic homeostasis by protecting against diet-induced obesity and against the condition of insulin resistance, predisposing to diabetes mellitus. Moreover, butyrate has an important anti-inflammatory effect, increasing the levels of Prevotella, which protect against Bacteroides, mediated negative effects on glucose metabolism. Additionally, short-chain fatty acids have been related to the improvement of the mucous protective layer by increasing the production of mucin and anti-microbial substances, decreasing the intestinal epithelium permeability, and also help the host defend against pathogenic bacteria, such as Clostridium difficile and Salmonella enterica.

The Mediterranean Diet has been reported to increase short-chain fatty acids and butyrate-producing bacteria, enhancing microbiota diversity whose benefits you’ve already heard of, increasing Bifidobacteria, and reducing Enterobacteria. As a result of these changes, a high adherence to the Mediterranean Diet delivers anti-inflammatory effects for the human body, with the reduction of toxins such as the level of bacterial lipopolysaccharides and of other mediators of inflammation circulating in the blood.

Recently, the analysis of gut microbial populations have shown two main variants, or enterotypes, in adults represented by Bacteroides and Prevotella. Bacteroide’s enterotype characterises individuals with high animal protein and saturated fat consumption. In contrast, the Prevotella enterotype was linked to carbohydrate-based diet. The enterotype identity remains stable. And even if rapid changes may occur after diet modification, the magnitude of the changes was modest and not totally sufficient to switch individuals between the enterotype clusters associated with protein fat and carbohydrates. Enterotypes are characterised by distinct digestive functions with preference for specific dietary substrates, resulting in SCFAs that may influence energy balance in the human host.

The term “fibre” is commonly used to describe indigestible carbohydrates. Although this designation is problematic, given that some fibres are not used by the gut microbes, such as cellulose, whereas other readily fermented carbohydrates fall outside of the definition of fibre, such as resistant starches. Furthermore, not all the fibre has the same effect on our gut microbiota. A specific subset of fibre has been termed prebiotic, as it selectively enhances growth of Bifidobacteria and lactobacilli.

There are functional differences between the enterotypes in relation to degradation of dietary fibres. In response to the introduction of fibre from grain bran, the Prevotella type individuals produce larger amounts of short-chain fatty acids. Those individuals lose weight and improve their metabolism with the consumption of whole grains. By contrast, the Bacteroides type individuals might respond better to soluble starch and pectin, fruits and vegetables, and improve their metabolism after the increase of Bifidobacteria by using prebiotic fibre.

The enterotype type potentially affects the individual’s ability to lose weight when following a specific diet. This has been demonstrated by a few scientific studies, but further research is needed to confirm these findings, which are interesting in view of the possibility in the future of a personalised nutrition based on the characteristics of our microbiota.